In order to be durable, beautiful or easy for cleaning and maintenance, most kitchen sinks in the market are metallic sinks made of stainless steel. Metallic sinks are used for washing kitchen utensils, dishes, fruit or vegetables. In order to avoid a clog in the drainage pipe, the water outlet of the metallic sink is connected with a drain connector, and a filter or a mesh is provided in the drain connector to filter residues in the drainage. In consideration of the cost, the metallic sink is made by sheet metal processing techniques to form a thin casing (thin wall) body. Because the wall of the bottom of the sink is thin, it is not easy to connect a drain connector at the outlet end of the water outlet. A conventional drain connector structure is disclosed in Taiwan Utility Model Patent No. M383593 titled “a drain connector structure of a kitchen sink”. The drain connector is disposed in the sink and extends out from the water outlet of the bottom of the sink. The rim of the drain connector is retained on the surface of the bottom of the sink to cooperate with a packing washer, so that the drain connector is located beneath the bottom of the sink. A locking ring is provided on an outer threaded section of the drain connector for fixing the drain connector to the water outlet. No matter how thin the packing washer is, there is still a gap between the rim of the drain connector and the surface of the bottom of the sink. There will be dirt and filth accumulated in the gap (it is easy to breed mold and bring odor). Furthermore, it is not easy for cleaning and maintenance to affect the appearance of the sink and hygiene.